SAINT PETERSBURG - THREE people were killed on Tuesday, including a child, when a car blew up in a suspected grenade blast near a metro station in Saint Petersburg, local officials said.

'A car exploded near the metro station Udelnaya at 8.55am (0555 GMT, 1.55pm Singapore time),' Andrey Alybayev, spokesman for the local branch of the emergencies ministry, told AFP.

'Three people were killed, including a three-year-old child.' He added that the explosion could have been caused by a 'grenade.' The car that exploded was a Russian Lada Priora, a standard passenger vehicle, the official said, adding that the incident was likely caused by a grenade.

The car was being used as an unlicensed private taxi of the type that are common in Russia and the driver had picked up a man, a woman and the toddler in the morning rush hour.

'The driver was taking passengers in the car. When the car approached the metro stop an explosion went off. A grenade exploded,' a security source told the RIA Novosti news agency.

Vladislav Piotrovsky, the local police chief, said that the explosion was not an act of terrorism.

'There was no crime. There was no act of terrorism. The grenade went off by itself. What is not clear is where the man got the grenade from.

'By the end of the day we will have a general picture of what happened,' he said, adding that the apartment of one of the victims was currently being searched.

The Interfax news agency said the driver survived the blast and was in a critical condition in hospital and the dead were a male and female passenger as well as the child.

Some reports said the dead were a family and Interfax said they were using their habitual driver who took the man to work every day.

Interfax quoted a source at the prosecutor's office as saying that bomb disposal experts working at the scene believed the blast was caused by a hand grenade of a type still to be determined.

The Udelnaya station is in the northern suburbs of the city north of the Neva river, around five stops from the centre of Saint Petersburg, Russia's second city.

The RIA Novosti news agency quoted an unnamed official saying the explosion took place about 50m from the entrance of the metro station.

It remained open even after the blast.

State television, without citing sources, said the grenade had been activated by one of the passengers when the car was passing the metro station.

Russia has been hit by a string of explosions in its major cities over recent years which have sometimes been blamed on score-settling in the murky Russian criminal underworld. -- AFP